When radiant energy from the sun strikes a surface some of it is absorbed, some of it is reflected into the atmosphere and lost, and some of it is transmitted.
One object of this invention is to provide a solar collector which can absorb essentially all of the radiant energy which reaches the earth's surface, that is, energy in the wavelength band of 0.3-2.0 microns. This absorbed energy can then be converted into heat or an electrical current via thermal-electric conversion techniques provided that the energy thus absorbed is not re-radiated into the atmosphere. Accordingly, it is desirable to utilize solar collectors which combine the advantages of high solar energy absorption with low emissivity, that is, low re-radiation.
Black bodies are known to absorb a significant amount of the energy in the solar spectrum and this would seem to indicate that such absorbers should be useful in energy collecting devices; however, black bodies also re-radiate most of their absorbed energy into the atmosphere in the form of infrared rays. Therefore, they are inefficient energy collectors.
Over the last few decades many different types of absorptive coatings or films have been devised so as to optimize solar absorptance and lower solar energy emission levels. These coatings have become known as "selective absorbers". They absorb radiant energy in the solar spectrum while at the same time they inhibit re-radiation into the atmosphere.
The principal factors affecting absorptance, emittance and thermostability are the physical and chemical properties of the absorber film, the nature of the substrate and the nature of the diffusion barrier or interlayer which lies between the said film and the said substrate.
The effectiveness of an absorptive film is measured in terms of its ability to absorb radiant energy from the sun, a property which is commonly described as "solar absorptance" (.alpha.). A good solar absorber is one having a solar absorptance level of at least 0.9. The next most important parameter, which becomes increasingly important at high collection temperatures particularly in systems having moderate concentration ratios, is hemispherical thermal emittance or emissivity (.epsilon.). The emissivity (.epsilon.) should be 0.1 or less and such levels have been achieved using certain highly polished metals such as silver, gold, copper and aluminum. However, gold is very expensive and certain other of these metals at high temperatures leave the absorber film in a metastable condition resulting in a rapid deterioration of said film.
Moreover, although silver, gold, copper and aluminum exhibit very low emissivities they also absorb very little solar energy. Aluminum also suffers the disadvantage of a low melting point.
In an effort to capitalize on the low emissivity of gold Robert C. Langley in U.S. Pat. No. 3,176,678 describes the construction of a solar energy collector in which a homogeneous receiver layer of gold and glass is joined to a metal substrate via a thin layer of a refractory oxide such as cerium oxide. The resulting collector is a highly effective absorber of solar rays suitable for high temperature operations.
Unfortunately, however, the receiver layer described by Langley (U.S. Pat. No. 3,176,678) is comprised of about 80-92% gold by weight and this high concentration of precious metal makes it prohibitively expensive from a commercial standpoint.
Attempts have been made to substitute less expensive metals for gold in the absorber film of Langley (U.S. Pat. No. 3,176,678). Silver, copper and aluminum, for example, have very low emissivities and they would appear to be suitable substitutes; however, these metals absorb very little solar energy and attempts to utilize them in solar collectors have been disappointing.
Also, attempts at substituting non-metal substrates for the metals described by Langley in U.S. Pat. No. 3,176,678 have not met with success. In fact, Langley eschews the use of quartz or glass as a substrate in solar collectors because of their fragility and relatively low softening temperatures.